The cheaters were way out ahead of the testers back then, taking stuff and blood doping in ways that either could not be detected yet, or by taking masking agents that could not be detected yet.

What they are doing now is clever, the ICF started something called the athlete passport, and other sports are catching on. They basically said "screw it, we're never going to be ahead of the drug companies, instead of looking for drugs we can't find, we'll just monitor the blood"

We all know about T/E ratios and how we still try to find steroids directly, but if the ratios get out of whack, we'll just assume you are on some kind of new designer steroid we cant see. They are now doing that with every biological cell, hormone, or substance that might occur naturally and which can help performance. We analyze the hell out of your blood, figure out what you are capable of naturally, then set ranges, for that specific person, based on what we've determined they can do. If they break those ranges in red blood cell production, or hormones which can improve performance, or whatever, we'll just say you cheated even though we don't know what you've done yet.

It seems to have been very effective, and race times went up after they started the athlete passports.

Catch any of the Tygart intv. on Dan Patrick?
Laid it all out; transfusions, plasma and saline expanders, masking agents, prior notice to testing...